The key point is that the AED (automatic electric defibrillator) will tell you if you have a shockable rhythm.
Many modern AED's can be used by untrained people, so if you see a cardiac arrest, find the nearest AED and deliver it and/or follow its (brief) instructions. Once you know this, you'll start tracking the last one you saw, and you'll find them more ubiquitous than you realized (and start advocating for one in your office).
(And if you are doing CPR, the breathing part is less important than the chest compressions. Blood flow is more important than oxygenation. But always/only follow current guidelines/training.)